There is substantial epidemiological evidence that being overweight or obese increases risk of colonic cancer. The data are stronger for men than for women and stronger for cancer of the left than right colon, with an overall doubling of risk. Data from two studies using Zucker rats support this notion. Diabetes is associated also with increased risk of colonic cancer, and this is directly supported by one animal study. Insulin and insulin-like growth factors are known to affect biological processes such as proliferation and apoptosis in vitro. Since obesity and hyperinsulinemia often co-exist in animal models and in humans, it has been difficult to determine their separate contributions to disease risk. In this application, we hypothesize that hyperinsulinemia is responsible for increased susceptibility of the overweight and obese to colonic cancer. We will use C57BL/6J mice to determine whether obese animals are more susceptible than their lean littermates to carcinogen-induced colon tumorigenesis. Initially, this will be done by comparing responses of wild-type versus Lepob mutants. In a second study, responses of lean, wild-type mice with normal weight and insulin levels will be compared to responses of mice that are both obese and hyperinsulinemic (induced by feeding a diet high in coconut oil) or hyperinsulinemic but not obese (induced by feeding a high-fructose diet). To determine the contribution that obesity in the absence of hyperinsulinemia makes to colonic tumorigenesis, plasma insulin levels will be reduced in Lepob mutants using metformin. Susceptibility to colonic tumorigenesis will be assessed by measuring aberrant crypt foci, crypt cell proliferation, and tumor incidence and multiplicity. We expect that hyperinsulinemia will stimulate colon tumorigenesis, whereas obesity in the absence of hyperinsulinemia will not. These studies will provide new knowledge that will be used to support a follow-up R01 application.